1. Field of the Invention
The present invention relates to apparatus for transferring fluid from a stationary reservoir to a rotatable member, and more particularly to means for conveying, controlling the flow, and varying the evaporating pressure and temperature of a cooling fluid from a reservoir to the rotor member of a dynamoelectric machine.
2. Description of the Prior Art
It is known that when certain materials, referred to as superconductors, are cooled to near absolute zero, they exhibit a complete loss of electrical resistance. Practical utilization of the zero resistance character of superconductive materials has been applied to great advantage in dynamoelectric machinery. For example, in a synchronous generator the use of a superconductive direct current field winding allows a considerable increase in the field magnetomotive force generated by a winding and greatly increased flux densities in the active air gap of the machine. This increase in flux density provides considerably increased power density and substantial reductions in the weight and volume of the machine. Thus, higher ratings for turbine generators can be obtained without prohibitive increases in frame size.
Superconductors which are suitable for such high current density and high field applications are subject to instabilities where a small disturbance in operating conditions can cause a quench. In particular, the superconductive effect will be quenched or lost unless the superconductors are maintained at very low temperatures. Therefore, it is imperative that adequate cooling arrangements be provided. Thus, when a winding or coil is formed of superconductive wires, provision must be made for bringing a coolant or refrigerant into intimate contact with the superconductor. For a dynamoelectric machine having a rotating superconductive field winding, provision must be made for transferring the cooling fluid from a stationary reservoir to the rotating field winding.
The transfer of a cryogen such as liquid helium from a stationary reservoir to the rotating field winding of a dynamoelectric machine has been accomplished in the prior art by a rotating transfer coupling apparatus. A necessary embodiment of the rotating transfer coupling apparatus is a rotating seal which functions to contain the cryogen and prevent the intrusion of air into the rotating member. If air intrudes into the cyrogenic space of the rotating member a slush is formed which obstructs the cooling flow passages, thus causing a quench. Rotating seals are normally operated above atmospheric pressure to prevent air intrusion.
The successful development of a rotating field superconducting dynamoelectric machine is dependent upon a reliable cryogen transfer system and proper management of the cryogen conveyed by the rotatable transfer coupling system. Proper cyrogen management includes several considerations: low temperature maintenance; flow balance; flow stability; cooling modes with a field winding; and, transient and quench requirements. In dynamoelectric machines utilizing two-phase cyrogen flow, i.e., a combination of a liquid and vapor, two-phase flow instabilities must be considered for proper helium management. The stability of the coolant flow in a rotating field winding is complicated by the pressurization of the helium coolant caused by the pumping action of the rotor as the coolant moves radially outward from its injection header on the machine axis. The enthalpy rise for this compression, in the adiabatic limit, is given by EQU .DELTA.h = (.omega..sup.2 r.sup.2 /2g)
where:
.DELTA.h is the incremental change in enthalpy;
.omega. is the angular velocity;
r is the radius of the fluid; and
g is the gravitational constant.
For a 3600 rpm machine with an injection pressure of 1 atmosphere, the critical pressure of 2.26 atmospheres would be reached at a rotor radius of 6 inches. For a rotor field winding radius greater than 6 inches, consideration must be given to the effect of passing the critical pressure of helium. If helium at one atmosphere and 4.2.degree. K is used as a coolant, the effects of the variations in physical properties on flow stability must be considered. These effects can be reduced by cooling the entering helium below 4.2.degree. K to a subcooled liquid state, thus ensuring single phase fluid pressurization that will be more stable than the two-phase process.
One of the major problems associated with two-phase systems is flow instability. Three types of known flow instabilities which are commonly encountered in rotating cryogenic systems are: Ledinegg instabilities in pressure drop oscillations; density wave oscillations; and, thermal acoustic oscillations. All of these instabilities are the result of the coexistence of two distinct phases of different density and transport properties. For further information concerning flow instabilities, see W. B. Bald et al., "Cryogenic Heat Transfer Research at Oxford-Part 2-Flow Boiling," Cyrogenics, pp. 179-197 (April 1974).
One means of reducing the possibility of flow instabilities is to pressurize the coolant above the point at which two phases can coexist, i.e., the critical pressure. For helium the critical pressure is relatively low (2.26 atmosphere) so this is not difficult. By heat exchanging with a helium bath at one atmosphere or less the supply temperature can be kept at 4.2.degree. K or below. However, the vacuum pump and heat exchanger necessary in such a precooling operation are expensive and heavy. Also another disadvantage of supercritical cooling is the fact that the latent heat of vaporization of the helium is no longer available for cooling. All heat absorption must result in a rise in coolant temperature. Therefore, to prevent an excessive rise in coolant temperature, the coolant mass flow must be substantially larger than that for boiling systems. Furthermore, two-phase cryogen flow is desirable for the simultaneous cooling of the field winding, the field winding support structure, the field winding excitation leads, and the radiation and electrical shields, all of which may require a different mass flow rate to maintain its temperature at an optimum value. Thus, it would be desirable to provide a rotatable cryogen transfer system which would simultaneously provide the supercooling of the cyrogen and also accurately control the cryogen liquid-vapor ratio.
Another disadvantage of existing rotating transfer systems is that they tend to compound the Ledinegg instability problem. Since the losses in the rotating transfer system are essentially constant, a flow perturbation which increases flow rate decreases the outlet temperature and hence pressure drop. Thus an increase in flow rate tends to cause a further increase in flow rate. Leading to a possibility of flooding the rotating member in two phase cryogenic systems.